Changes in circumpapillary retinal nerve fiber layer thickness after vitrectomy for rhegmatogenous retinal detachment

The study aimed to evaluate the long-term changes in circumpapillary retinal nerve fiber layer (RNFL) thickness after vitrectomy for rhegmatogenous retinal detachment (RRD) repair. A total of 33 eyes of 33 patients were enrolled. By using optical coherence tomography, the circumpapillary RNFL thickness was measured before surgery and 1, 3, 6 months and 1, 2, 3 years after surgery and compared with the preoperative value. The effect of duration, location, and extent of RRD on RNFL thickness change was evaluated. There was a significant increase of circumpapillary RNFL thickness at the 1-month, 3-month [except in the nasal superior sector (P = 0.627)], and only in the nasal inferior sector at 6-month (P = 0.010) follow-up compared with the baseline value (all Ps < 0.05). No significant differences were observed 1, 2, and 3 years after the surgery (P > 0.05). The duration, location, and extent of detachment did not reveal significant correlations with RNFL parameters (P > 0.05). Circumpapillary RNFL thickness in eyes with RRD after vitrectomy demonstrated a transient increase during the early postoperative period. This increase was not associated with duration, location, and extent of RRD. At 3 years following surgery, no RNFL thinning or thickening was observed.

. Preoperative baseline clinical characteristics of eyes with retinal detachment (n = 33). Values are presented as number, mean ± standard deviation, or median (interquartile range). SD Standard deviation, logMAR Logarithm of the minimum angle of resolution, IOP Intraocular pressure.

Clinical parameters Observed values
Age (years, mean ± SD) 54. 2 Table 2. Visual acuity and intraocular pressure change after surgery. Data are presented as median (interquartile range). logMAR Logarithm of the minimum angle of resolution. *Wilcoxon signed-rank test (compared with baseline value).
Visual acuity (logMAR) P value* Intraocular pressure (mmHg) P value*  Table 3). Mean relative RNFL thickness change ranged from 4.9% (TS sector) to 13.3% (N sector) at the 1-month follow-up, from 0.9% (NS sector) to 9.4% (NI sector) at the 3-month follow-up, and from 0.6% (T sector) to − 3.8% (NS sector) at the 3-year follow-up (Fig. 1). The circumpapillary RNFL thicknesses of the fellow eye at 6 months, 1 year, 2 years, and 3 years after the surgery did not show significant change compared with baseline values in all sectors (all Ps > 0.05). When compared with the eyes with RRD, fellow eyes had thinner RNFL in NI (from baseline to 2-year follow-up) and N sectors (from baseline to 3-year follow-up, all Ps < 0.05, Table 4).
Affecting factors for circumpapillary retinal nerve fiber layer thickness change after surgery. Comparison of RNFL thickness and relative RNFL thickness change at each visit between eyes with superior RRD and inferior RRD and between eyes with macular-on RRD and macular-off RRD showed no significant differences at all sectors and visits (all Ps > 0.05). The duration, extent of detachment, and final BCVA did not show significant correlations with SD-OCT parameters (all Ps > 0.05).

Discussion
In the present study, circumpapillary RNFL thickness in eyes with RRD after vitrectomy showed a transient increase during the early postoperative period (within 6 months) regardless of duration, location, and extent of RRD. This increase disappeared after this point and no RNFL thinning was found compared to the preoperative values until 3 years after the surgery. To the best of our knowledge, this is the only prospective long-term study investigating and comparing circumpapillary RNFL thickness changes after vitrectomy to the preoperative values.
To identify the effect of RRD on retinal layers, observation of retinal thickness changes after the development of RRD without surgical intervention may be needed. However, given that untreated RRD can induce severe visual loss, this may be limited. In human eyes, several studies have reported RNFL thickness changes after the surgical repair of detachment by using non-invasive imaging devices, such as scanning laser polarimeter 3 or OCT [4][5][6][7][8][9][10][11] . A study using time-domain OCT reported that RNFL thinning was not found within 6 months 4 . However, RNFL was thinner in the area of detached retina than fellow eyes 12 and 24 months postoperatively 4 . The authors suggested that this may have been caused by atrophy of RGCs and a subsequent decrease in RNFL thickness 4 . In contrast, other studies found no significant change 3 or increase in RNFL thickness after repair of the detachment 5-7,10,11 . Our study also revealed a transient increase in RNFL thickness during the early postoperative period (within 6 months) that may be explained by RNFL edema, retinal ischemia, or Müller cell proliferation associated with retinal detachment [3][4][5][6][7]10,11 .
Transient circumpapillary RNFL thickness increase after repair of RRD may be explained by either the effect of RRD and/or the result of surgical procedures to repair it. We hypothesized that if RRD itself causes RNFL thickness change, the duration, location, or extent of RRD may be associated with the location or amount of RNFL thickness change. Regarding the topographic correlation between RRD location and RNFL thickness change, a study using time-domain OCT analyzed the clock-hour sector RNFL thickness of detached area and revealed that RNFL thickness at the detached area was lower than corresponding area of the fellow eye 4 . However, given that the pathway of RNFL is not linear, this method may not reflect the real topographic correlation 15 . The RNFL in superior or inferior hemisphere does not cross the horizontal raphe. Therefore, we compared the Table 3. Circumpapillary retinal nerve fiber layer thickness changes after surgery. Significant values are in bold. Data are presented as median (interquartile range). G Global area, TS Temporal superior, TI Temporal inferior, NS Nasal superior, NI Nasal inferior, T Temporal, N Nasal, PMB Papillomacular bundle. *Denotes a significant difference (P < 0.05). † Wilcoxon signed-rank test (compared with baseline value).
Global P value † TS P value † T P value † PMB P value † TI P value † NI P value † N P value † NS www.nature.com/scientificreports/ RNFL thickness change pattern between eyes with RRD in the superior and inferior hemispheres and found no significant association of the location of RRD with RNFL thickness change patterns. A study reported that RNFL thickness increase after retinal detachment was associated with increase in the duration of detachment; this may be caused by the proliferation of Müller cells, which may be more prominent in longstanding detachment 3 . If this were true, eyes with a longer duration of detachment may have a greater preoperative RNFL thickness. However, we could not find any significant correlation between the duration of detachment and OCT parameters.  It has been reported that transient RNFL thickness increase was also found after vitrectomy for macular hole or epiretinal membrane [12][13][14] . This may be caused by trauma to the inner retina, fluctuations of IOP, toxicity to the retina from light, dye, fluids, or gases, and inflammatory responses during the surgery [12][13][14] . During the vitrectomy for RRD, similar procedures and devices are applied to the eye. Therefore, we think that comparable mechanisms to vitrectomy for macular hole or epiretinal membranes may play a role in eyes with RRD during the vitrectomy.
When RNFL thickness of eyes with RRD was compared with the RNFL thickness of unaffected fellow eyes, no significant difference was found 1 year after the surgery, with the exception of RNFL in NI and N sectors; RNFL thicknesses in these sectors of affected eyes were greater than in the fellow eyes. This may be explained by a greater RNFL thickness of affected eyes in these areas before the development of RRD or a greater effect of vitrectomy on these areas compared to other areas. A previous study also reported that RNFL thickness increase after vitrectomy was most prominent in the nasal area 13 . Further studies with similar topographic characteristics of the RNFL between affected and unaffected eyes at preoperative baseline visit are required to elucidate on this issue.
Most of the previous studies with RRD evaluated RNFL thickness compared to the unaffected fellow eyes or eyes of healthy control participants [3][4][5][6][7]11 . Although the fellow eye may be a useful control, it has been reported that inter-eye differences in RNFL thickness exist within the same individual 16,17 . Therefore, preoperative baseline values of the same eye may be a better control than the fellow eye or healthy controls. A possible limitation of comparison with the preoperative baseline value is the presence of OCT segmentation errors or artifacts caused by RRD. To minimize this confounding effect, we only included eyes without RRD within the scan circle area centered on the optic nerve head.
It has been suggested that a thin RNFL may reflect worse visual function after RRD repair 4,8 . In the present study, final BCVA was not significantly associated with any RNFL thickness parameters. Previous studies also reported that thickness of the inner retinal layers did not show significant correlation with vision 7,10 . We investigated only RNFL thickness values measured where a 3.46-mm scan circle passes; RNFL thickness change distal to the 3.46-mm scan circle was not evaluated. It has been reported that when assessing RNFL thickness change in glaucoma, progression was most frequently found in the inferotemporal area outside of the 3.46-mm scan circle 18 . Therefore, we may speculate that RNFL thickness analysis in eyes with RRD including RNFL thickness values distal to the 3.46-mm scan circle may provide more information. In the present study, only thickness values were investigated. Assessment of structural integrity (i.e., presence of irregularity, discontinuity, or cystic change) other than thickness change should be considered. Visual function can be evaluated not only by BCVA but also by visual field test or electroretinogram. Further studies with these modalities may be needed. To conduct a prospective long-term follow-up study, it was difficult to enroll participants with a large number. A relatively smaller number of participants in the current investigation compared to the previous studies with 33 to 67 eyes 4-7,9,10 may have caused a lower power to detect a clinically relevant change in the RNFL.
In conclusion, circumpapillary RNFL thickness in eyes with RRD after vitrectomy showed transient increase during the early postoperative period. This increase was not associated with duration, location, or the extent of RRD. Until 3 years after surgery, no RNFL thinning was found and the RNFL thickness values were not correlated with visual acuity after the surgery. Table 4. Circumpapillary retinal nerve fiber layer thickness in fellows eyes of the eyes with retinal detachment. Significant values are in bold. Data are presented as median (interquartile range) of fellow eyes. *Denotes a significant difference (P < 0.05). † Wilcoxon signed-rank test (comparison between eyes with retinal detachment presented in Table 3 and fellow eyes). G Global area, TS Temporal superior, TI Temporal inferior, NS Nasal superior, NI Nasal inferior, T Temporal, N Nasal, PMB Papillomacular bundle.

Materials and methods
Participants. This prospective cohort study protocol was approved by the Institutional Review Board of Hangil Eye Hospital. The study was carried out in accordance with the tenets of the Declaration of Helsinki. Informed consent was obtained from the participants. Among the patients with RRD who underwent PPV to repair detachment at Hangil Eye Hospital between February 2017 and June 2018, those aged ≥ 18 years and with RRD in the affected eye with a healthy fellow eye, no history of previous retinal surgery in either eye, absence of other retinal diseases (i.e., epiretinal membrane, diabetic retinopathy, retinal vein occlusion, and macular degeneration), optic neuropathy (i.e., glaucoma), or any condition with media opacity that can cause insufficient spectral domain optical coherence tomography (SD-OCT) image quality in either eye, were consecutively enrolled. For the eyes with RRD, the location of the detachment was subdivided into superior, nasal, inferior, and temporal quadrants in accordance with the SD-OCT circumpapillary RNFL quadrant classification (dotted lines in Fig. 2). When it was present across multiple quadrants, the quadrant with the greatest area of detachment was selected. The extent of RRD was defined as the angular distance between the lines connecting the margins of the RRD to optic nerve head center (angle α in Fig. 2). The extent was presented as clock-hour area.
Surgical methods. A single surgeon (DDH) performed all the surgeries. A 3-port 25-gauge scleral tunnel transconjunctival sutureless PPV was performed using the Constellation Vision Surgical System (Alcon Surgical, Fort Worth, TX, USA). It was performed under 2% lidocaine sub-Tenon anesthesia with monitored anesthesia care. Phacoemulsification with posterior chamber intraocular lens implantation was also performed in patients with cataract. After the core vitrectomy, the peripheral vitreous was thoroughly shaved using scleral indentation. The subretinal fluid was drained through the preexisting peripheral retinal breaks after filling perfluorocarbon liquid (PFCL, Decaline® 7 mL, FCI SAS, Paris, France) up to the posterior edge of the break. Endophotocoagulation around the breaks on the peripheral retina was then applied. Following PFCL removal and fluid-air exchange, the surgery was completed after confirmation of the reattachment of retina and absence of any epiretinal membrane and macular hole using the intraoperative OCT (RESCAN 700, Zeiss, Germany) in all patients. After that, in all cases, gas endo-tamponade (C3F8, 10% perfluoropropane) was performed and those patients were then recommended to maintain a prone position for one week after surgery.
Ophthalmic examinations. Slit-lamp examination, fundoscopy, BCVA assessments presented as a logarithm of the minimum angle of resolution (logMAR), IOP measurements using Goldmann applanation tonometry, and circumpapillary RNFL thickness measurements were performed at the baseline visit before surgery and 1 month, 3 months, 6 months, 1 year, 2 years, and 3 years after surgery. The RNFL thickness of the fellow eye was measured at the baseline visit before surgery and 6 months, 1 year, 2 year, and 3 years after surgery. Axial length was measured by IOLMaster 700 (Carl Zeiss Meditec AG, Jena, Germany) at the baseline visit. Circumpapillary RNFL thickness was measured using SD-OCT (Spectralis OCT; Heidelberg engineering, Heidelberg, Germany). After proper alignment, the optic disc was centered to the SD-OCT scan and images were acquired. By using an equipped software (Spectralis Nsite Axonal Analytics Software; Heidelberg Engineering, Heidelberg, www.nature.com/scientificreports/ Germany), circumpapillary RNFL thickness was automatically segmented and presented in six sectors: temporal (T, 315°-45°), temporal superior (TS, 45°-90°), nasal superior (NS, 90°-135°), nasal (N, 135°-225°), nasal inferior (NI, 225°-270°), and temporal inferior (TI, 270°-315°), together with the papillomacular bundle (PMB, 338°-8°) thickness. The global (G) RNFL thickness was obtained by averaging the total 360° RNFL thicknesses. Given that detached retina within the SD-OCT scan area may cause a segmentation error, only eyes with retinal detachment not involving the 3.46-mm scan circle centered on the optic nerve head and eyes without segmentation errors or artifacts were included.
Statistical analyses. Statistical analyses were performed using a commercially available software (SPSS Statistics version 23; IBM Corp., Armonk, NY, USA). Distribution of variables was evaluated by Kolmogorov-Smirnov test. Variables with a normal distribution were presented as mean and standard deviation, while variables without a normal distribution were presented as median and interquartile range, and categorical variables were described as frequencies. Visual acuity, IOP, and SD-OCT thickness values did not show a normal distribution. Thus, comparison between the baseline visit and each follow-up visit and the comparison between the affected eye and fellow eye at each visit were performed by the Wilcoxon signed-rank test. Comparison of axial lengths in the affected eye and fellow eye was performed by the paired t-test.
To investigate the effect of RRD location on RNFL thickness change, the eyes were subdivided into eyes with superior, nasal, inferior, and temporal RRD based on the RRD location. To identify the effect of macular status on RNFL thickness change, the eyes were subdivided into macular-on and macular-off groups. RNFL thickness and relative RNFL thickness change (%), presented as (RNFL thickness at follow-up visit-RNFL thickness at baseline visit/RNFL thickness at baseline visit) × 100, at each visit were compared between the groups using the Mann-Whitney U test. Correlation between the duration and extent of detachment, final BCVA, and SD-OCT parameters were assessed by Spearman's correlation coefficient. A P value < 0.05 was considered statistically significant.

Data availability
The data are not available for public access because of patient privacy concerns, but are available from the corresponding author on reasonable request.